Automatic direction finders having a crossed-loop antenna circuit with a goniometer inserted into the circuit are known. The output from the goniometer is modulated with a low frequency signal, and the modulated signal is combined with the output from a vertical antenna in a 90.degree. phase shift relation therewith. Thereafter, the resultant signal is amplified and detected by a receiver; the detected signal is fed to the driving winding of a servomotor, and at the same time the aforementioned low frequency signal for modulation is applied to the energizing winding of the servomotor which is mechanically coupled to the goniometer to indicate the direction of an incoming electromagnetic wave by the angular position of the pointer of the goniometer.
It is also known that a crossed-loop antenna produces two outputs each of which is modulated with a respective one of two low frequency signals that have the same frequency but are 90 degrees out of phase with each other, respective to one of a pair of modulators. The modulated signals and the output from a vertical antenna are all combined into a signal after the vertical antenna output is phase shifted by 90 degrees. The combined signal is then amplified and detected by a receiver. Thereafter it is passed through a filter which filters out all the frequency components except for the frequency for modulation. Then, the phase of the reference frequency signal for the aforementioned low frequency for modulation is compared with the phase of the output frequency from the filter to produce a phase difference angle signal which is employed to indicate the direction of a received electromagnetic wave.
The prior art direction finders as above defined have certain shortcomings. Specifically, if the aforesaid direction finders are installed on a ship or the like, there unavoidably exist many objects such as masts and stays near the antennas which will disturb electromagnetic waves. Particularly as a frequency to be measured becomes closer to the VHF band, high frequency groundings of a vertical antenna and the related elements employed for determining sensing become less stable; and, therefore, the amplitude and phase of a signal in the vertical antenna vary substantially with the direction of incoming electromagnetic waves as a result of being affected by masts and stays, thereby frequently obscuring the sensing and increasing error.